Field of the Invention
The present invention relates to a flow control apparatus for a positive-displacement pump such as rotary-vane pump, plunger pump, gear pump, and more particularly to the flow control apparatus for keeping a flow rate of fluid discharged from the pump at the high rotational speed which is lower than a flow rate of fluid discharged from the pump at the low rotational speed.
Generally, a positive-displacement pump, for instance, rotary-vane pump, installed in automotive vehicles which are driven by engines, is operated by the engine acting as power source and utilized as fluid pressure source for supplying hydraulic fluid to actuators of various hydraulic equipment, for instance, power steering systems.
Among various types of the power steering systems for assisting torque generated in manual steering by using hydraulic fluid, there is one type adapted to provide relatively great steering assistance at low vehicle speed and relatively small steering assistance at high vehicle speed. This is because the steering is stable at the high vehicle speed. A positive-displacement pump mounted to such type of the power steering system is required to discharge a high flow rate of fluid at the low rotational speed, i.e., at the low vehicle speed, and a low flow rate of fluid at the high rotational speed, i.e., at the high vehicle speed. For this reason, there have been recently proposed flow control apparatuses adapted to control a flow rate of fluid discharged from the pump and exhibit the aforementioned characteristic of the flow rate of fluid with respect to the rotational speed of the pump. Description of the Related Art One example of the flow control apparatuses as proposed is disclosed in German Patent Application First Publication No. DE4433598A1. The apparatus includes a variable flow control valve disposed within a discharge passage communicating with the discharge side of a positive-displacement pump, and a flow control circuit cooperating with the discharge passage to permit fluid to return the suction side of the pump. The flow control circuit includes a drain valve adapted to drain the fluid discharged from the pump in response to a difference between pressures upstream and downstream of the variable flow control valve. The variable flow control valve is operative to vary a flow of fluid that is discharged from the pump and delivered to actuators through the discharge passage. The variable flow control valve includes a spool facing the fluid discharged from the pump and moveable to vary an opening area of the discharge passage, and a spring biasing the spool so as to increase the opening area of the discharge passage. The drain valve and the variable flow control valve cooperate to control the flow rate of the discharged fluid passing through the discharge passage.
In this conventionally known apparatus, when the rotational speed of the pump increases beyond a set value up to a greater value than the set value, the drain valve and the variable flow control valve cooperate to reduce the flow rate of fluid passing through the discharge passage down to a predetermined value. Subsequently, when the rotational speed of the pump exceeds the greater set value, the drain valve and the variable flow control valve cooperate in order to keep the flow rate of fluid of the predetermined value. Under such condition as the rotational speed of the pump exceeding the greater set value, the flow rate of fluid discharged from the pump becomes much higher than the flow rate of fluid drained from the drain valve. However, the known apparatus tends to cause undesired increase in flow rate of fluid passing through the discharge passage over the predetermined value. When the pump is operated at the high rotational speed beyond the greater set value, the characteristic of the flow rate of fluid passing through the discharge passage becomes unstable due to the flow rate of fluid increasing as the rotational speed of the pump rises. This leads to decrease of operating accuracy of actuators and then hydraulic equipment to which the fluid discharged from the pump is supplied via the discharged passage.
It is an object of the present invention to provide a variable flow control apparatus for a positive-displacement pump that is capable of achieving a desirably stable characteristic of the flow rate of fluid discharged from the pump at the high rotational speed.